The present invention relates to optical technology and more particularly to a method and system for providing a wavelength locker which is cost effective and capable of very precisely locking a wavelength of an optical signal to within particular limits.
A variety of optical technology, such as communication via optical fibers, utilizes optical signals of a particular wavelength or wavelengths. Wavelength lockers ensure that an optical signal has a particular wavelength, xcexC, to within particular limits. Conventional wavelength lockers translate a change in wavelength to a change in intensity using a variety of configurations and components.
One conventional wavelength locker utilizes a pair of Fabry Perot (xe2x80x9cFPxe2x80x9d) etalon filters. A reference optical signal from a light source desired to be locked in to the wavelength xcexC is provided to a partial reflector. The partial reflector reflects a first portion of the optical signal and transmits a second portion of the optical signal. The first and second portions of the optical signal are provided to the first and second FP etalon filters, respectively. The response of each FP etalon filter changes with wavelength. When the wavelength of the optical signal is xcexC, the FP etalon filters transmit the same intensity. When the wavelength of the optical signal drifts from xcexC, the FP etalon filters transmit different intensities. Photodetectors receive light transmitted by the FP etalon filters. The difference in intensities when the optical signals drift from xcexC is used to lock the light source to wavelengths within a particular range of xcexC.
Another conventional wavelength locker also uses an etalon filter. A reference optical signal from a light source is provided to the filter. The filter reflects a portion of the optical signal and transmits a portion of the optical signal. The transmission and reflection responses of the etalon filter change in opposite directions with the wavelength of the optical signal provided to the filter. When the wavelength of the optical signal is xcexC, the reflected and transmitted intensities are the same. When the wavelength of the optical signal drifts from xcexC, the reflected and transmitted intensities differ. Photodetectors receive the transmitted and reflected portions of the optical signal. The difference in intensities when the optical signal drifts from the desired wavelength xcexC is used to control the light source to produce light having wavelength xcexC within the particular limits. Yet another conventional wavelength locker is based on a holographic grating.
Although the conventional wavelength lockers function, one of ordinary skill in the art will realize that the FP etalon filters used in some of the conventional wavelength lockers are expensive and difficult to manufacture. The holographic grating technology is both temperature sensitive and very expensive. Methods have been published to replace etalon or holographic grating with dielectric thin film interference filters. However, the filters may also induce losses in transmitted light that depend on polarization. These polarization dependent losses may adversely affect the conventional wavelength locker""s ability to accurately determine wavelength and, therefore, lock in on the particular wavelength.
Accordingly, what is needed is a system and method for providing a cost-effective, high precision wavelength locker, for example a wavelength locker that is suitable for dense wavelength division multiplexer applications. The wavelength locker produced should also be reliable and stable over a range of temperatures. The present invention addresses such a need.
The present invention provides a method and system for stabilizing a wavelength of an optical signal to within a particular range, otherwise known as locking the wavelength. In one aspect, the method and system comprise providing a partial reflector, a first filter, and a second filter. The partial reflector is for receiving an optical signal, transmitting a first portion of the optical signal, and reflecting a second portion of the optical signal. The first filter is for receiving the first portion and transmitting a third portion of the optical signal. The first filter includes a first axis and is capable of being tuned by rotation around the first axis to transmit the third portion including a particular wavelength. The second filter is for receiving the third portion and transmitting a fourth portion of the optical signal. The second filter includes a second axis substantially perpendicular to the first axis and is capable of being tuned by rotation around the second axis to transmit the fourth portion including the particular wavelength. In this aspect, the first filter may have a first transmission spectrum and the second filter may have, a second transmission spectrum substantially similar to the first transmission spectrum. The particular wavelength may be located at first and second points on the first and second transmission spectra, respectively, to maximize a range in which the particular wavelength can be provided and reduce polarization dependent loss. In another aspect, the method and system comprise separating an input signal into first and second portions and filtering the first portion to provide a third portion of the input signal. The third portion has a first polarization dependence. The method and system also comprise filtering the third portion to provide a fourth portion of the input signal. Filtering the third portion includes compensating for the first polarization dependence. Thus, the fourth portion has a second polarization dependence that is less than the first polarization dependence.
According to the system and method disclosed herein, the present invention provides a wavelength locker which is relatively inexpensive and precise thereby increasing overall system performance.